1. Field of the Invention
The present invention relates to a video encoder and its decoder and, more particularly, to a video encoder and its decoder realizing improvement in coding efficiency of a super high definition video by optimally determining division and size of a macroblock used for motion prediction.
2. Description of the Related Art
Conventionally, as a method of improving coding efficiency of a motion predicting unit in video encoding, a method of optimally determining the upper limit of the size of a macroblock (MB) used for motion prediction is described in, for example, Japanese Patent Application Laid-Open No. 2006-339774 and “Improved coding scheme for super high definition videos by using H.264 high profile coding framework with extended macroblock size”, The Journal of the Institute of Image Information and Television Engineers, Vol. 61, No. 5, pp 665-673 (2007).
In Japanese Patent Application Laid-Open No. 2006-339774, a method of classifying the MB size into three categories of “large”, “medium”, and “small” is proposed. Considering rough tendency of the MB size in results of coding in the previous picture, when a usage ratio of the category “large” is higher than a first threshold and a usage ratio of the category “small” is lower than a second threshold, the upper limit of the MB size of the picture is increased. On the other hand, when the usage ratio of the category “large” is lower than the first threshold and the usage ratio of the category “small” is higher than the second threshold, the upper limit of the MB size of the picture is decreased.
A method of employing an MB size with small distortion obtained by using a distortion rate curve for use in rate control of an encoder as the maximum MB size is disclosed in the “Improved coding scheme for super high definition videos by using H.264 high profile coding framework with extended macroblock size”, The Journal of the Institute of Image Information and Television Engineers, Vol. 61, No. 5, pp 665-673 (2007).
According to the conventional techniques, the maximum MB size to be applied to the picture can be roughly determined in consideration of tendency of the MB size in the result of coding in the previous picture.
However, as one of features of a super high definition video, it is known that a fluctuation in a local material characteristic is large. Consequently, in many cases, it is difficult to characterize a picture by one coding characteristic. When any of the categories of large, medium, and small limited by the maximum MB size is just allocated to a picture, a problem occurs such that an effective coding mode cannot be sufficiently covered.